In an ideal television system the light output produced by a kinescope would be linearly related to the light applied to a camera pick-up tube. In practical systems, neither the camera tube or the display tube are linear devices. In other words, the signal voltage produced by a camera tube is not linearly related to the light that is detected and the light produced by a kinescope is not linearly related to the cathode drive voltage applied to it. The relationship between light input and signal output for the camera tube, and the relationships between signal input and light output of the picture tube, are both commonly expressed by the term "gamma" which, simply stated, is the exponent or "power" to which an input function (X) is raised to produce an output function (Y). If, for example, an input function X is raised to the first power (gamma=1) to produce an output function, then the two functions are said to be linearly related. If the output varies as the square of the input function, the value of the exponent (gamma) is equal to "2". If the output varies as the square root of the input function, the "gamma" or exponent equals 0.5. Gamma, in other words, is simply a measure of curvature of a transfer function,
FIG. 1 shows the gamma of various aspects of a video signal transmission system, with curve 100 representing the transfer characteristic of the transmission side, curve 102 representing the transfer characteristic of the picture tube (kinescope or "CRT"), and curve 104 representing the overall transfer characteristic. The transmitted video signals of the NTSC, PAL and SECAM television standards have a gamma of about 0.45 to 0.5 while the picture tube (kinescope) of color television receivers have a gamma of about 2.8 to 3.1. As a result, the overall transfer curve (light into the camera to light output from the picture tube) is not linear and the overall gamma is, in practice about 1.35 instead of a unity (1.0) gamma. This implies that the exponential transfer characteristic of the picture tube is not fully compensated, leading to compression of dark picture portions of the display. Such compression causes picture details near black to be lost, and colored areas to fade to black. Concurrently, whites are excessively amplified with respect to the dark portions to the point of often reaching picture tube saturation and blooming.
A linear overall transfer characteristic avoids the problem of black compression and can be obtained by an additional gamma correction of about 0.8 in each of the red, green, and blue (R, G and B) signal processing circuits in the television receiver. However, picture tubes have a relatively small dynamic range of light output which can not be enlarged without reaching picture tube saturation causing blooming. Therefore, gamma correction to increase amplification of dark image areas can cause a signal compression of the high signal whites. This is illustrated in FIG. 2A showing a partially gamma corrected ramp signal in which the gain for signals near black level is increased. It is desirable, however, that peak white be kept at the same level as in the uncorrected case, the dashed line, to avoid picture tube blooming. For this to occur, the slope of the upper portion of the ramp signal may be reduced as shown in FIG. 2B. This corrects the black compression problem while avoiding the problem of "blooming" (excessive whites).
Reducing the upper portion of the ramp signal to avoid blooming, however, can create another problem. The viewer perceives the reduced signal as a lack of contrast in gray to white picture areas resulting in "washed out" appearing pictures. In such an event, the improvement of contrast of low-brightness portions of the image by gamma correction is obtained at the expense of high brightness contrast deterioration.
There are, generally speaking, two conventional approaches to gamma correction. One approach is to apply non-linear processing to the video signal in the driver circuitry as exemplified, for example, by Haferl et al. in U.S. Pat. No. 5,083,198 which issued Jan. 21, 1992. In an embodiment of the Haferl et al. apparatus, a video signal is divided into low and high amplitude portions, the latter are high pass filtered and then the original video signal, the low amplitude portion and the high pass filtered high amplitude portion are combined for application to a kinescope. Images displayed include gamma correction for black to gray picture areas and boosted detail for gray to white picture areas.
The other approach to gamma correction is to apply linear processing to the video signal and rely upon the non-linear impedance characteristics of the kinescope cathode for gamma correction as exemplified, for example, by Furrey in U.S. Pat. No. 4,858,015 which issued Aug. 15, 1989. In an embodiment of the Furrey apparatus a video signal is linearly amplified in a cascode amplifier. The amplifier output impedance is reduced by coupling the amplifier load resistor to the input of a voltage follower amplifier comprising a cascade complementary emitter follower buffer amplifier. The output of the voltage follower amplifier is coupled to the kinescope cathode via a parallel connection of a resistor and a capacitor. The resistor, in combination with the non-linear resistive portion of the cathode impedance, provides gamma correction. However, the resistor, in combination with the stray capacitance of the cathode, creates an undesirable frequency response pole at a relatively low frequency (i.e., it acts as a low pass filter). This tends to reduce the high frequency detail of displayed images. The inclusion of a by-pass capacitor in parallel with the resistor tends to restore the high frequency response by by-passing high frequency components around the gamma correction resistor. The complementary emitter follower (buffer) amplifier provides a low impedance source for driving the by-pass capacitor.